This invention relates to an improvement in the electrostatic powder coating apparatus and more specifically to a powder coating apparatus which is characterized by comprising a face-silent discharge electrode capable of generating a silent discharge in the form of a face which is arranged to be separated from and to confront a substrate, means for supplying powder coating material between them, and means for impressing a voltage between the face-silent discharge electrode and the substrate.
In electrostatic powder coating apparatus of the prior art, a powder coating material suspended in the air by means of compressed air is injected against a substrate from a nozzle of a spray gun disposed to be separated from and to confront the substrate. Simultaneously, a high DC voltage is impressed on the nozzle of the gun to generate a corona discharge there. The powder in the coating space is charged by virtue of said corona discharge and lines of electric force from the tip of the gun to the substrate. Charged coating material is transferred to a surface of the substrate by a coulomb force caused by the electric field between the tip of the gun and the substrate to form a coating layer of powder material on the surface of the substrate. Thereafter, the resulted layer is baked by heating. This is a usual way to obtain a coating on a substrate.
In the prior process of electrostatic powder coating, currents caused by a gun concentrates at said gun. Thus, the formed powder coating material layer is thick in front of the gun and becomes thinner and thinner as the coated place is more and more distant from the gun. Accordingly, an evenly thick layer is difficult to obtain unless the gun is moved. When the gun is moved in the plane parallel to the surface of the substrate in pursuance of obtaining an even coating, the over spray of powder in the both sides increases and it is inevitable that the coating efficiency remarkably decreases.
In addition, as a result of the currents concentrating in front of the gun, the voltage drop by ionic currents at the central part exceeds the spark voltage of the layer of powder having a high resistivity, such as those employed now, before the sufficient thickness of layer is obtained in the peripheral parts. Consequently, a back ionizing phenomenon arises to form pinholes and surface roughness in the powder layer. This is a serious drawback of the prior art powder coating systems.
In prior powder coating systems, even though the efficiency of the gun itself attains a value of 80 - 90%, ignoring the shape of a substrate, it is usual that the coating efficiency attains a value of not more than sixty percent in an actual coating process. The current and the field concentrate at the front of the gun. Moreover, a corona discharge is generated at the tip of the gun. Thus, the electric field remarkably concentrates at this place. A powder layer tightly adheres to the tip of the gun by the so-called gradient force to change the voltage-current characteristic. Accordingly, the coating process cannot be stably continued unless the operation is frequently interrupted and the tip of the gun is cleaned. This is a serious drawback of the prior process for powder coating.
Further, when the physical shape of the gun is determined and a voltage of a definite value is impressed on the gun in the case of the prior powder coating process, the obtained current has unequivocally a definite value. It is impossible to mutually independently select a value of current and a value of voltage depending on the properties of a powder used. This is responsible for the impossibility of improving properties of the coating. An electrode having a sharp tip is typically employed for generating a corona discharge according to prior electrostatic powder coating systems. A spark due to the corona discharge can run towards the substrate and when a combustible coating material powder is employed, the coating apparatus is in danger of igniting and exploding.